Thermoacoustic instabilities in combustors are known to be precursor to flame-out or damage to engine components. Accurate quantification of combustor pressure dynamics for the primary purpose of experimental validation of computational fluid dynamics (CFD) codes requires the use of robust, reliable and sensitive pressure sensors that can resolve sub-psi pressure levels in high temperature environments (e.g., a combustor of a Brayton cycle heat engine). Current pressure sensors are placed feet away from a test article while pressure is transmitted through a tube. Since the tube is an acoustic filter, it imposes limitations on the frequency bandwidth of the thermoacoustics. Water-cooled pressure sensors are used in order to reduce the length of the tube and increase the bandwidth. However, the flow of the coolant around the sensor introduces turbulence noise, which tends to corrupt the signal.
Current uncooled microfabricated piezoresistive SiC pressure sensors produced by NASA Glenn Research Center are capable of operating reliably at 600° C. without these water cooling jackets. However, when used to quantify combustor thermoacoustic instabilities, while the SiC pressure sensors survived the high temperature and measured instabilities, these diaphragms (i.e., force collectors) are not thin enough to be sensitive in resolving sub-psi pressures. Existing instability prediction models have high uncertainty margins at high temperature.
30 microns is the thinnest diaphragm achievable with conventional reactive ion etching (RIE) processes. This diaphragm thickness precludes its use for sub-psi pressure measurement with high fidelity. Accordingly, an improved process for fabricating SiC diaphragms may be beneficial.